The production of bi-material plastic parts has been a widespread technique for many years. The field of application is very broad, encompassing technical parts, for example, for automobiles, electronics, household appliances, etc., articles for the cosmetics, perfume, personal care industry, etc. Likewise, the type of raw materials, i.e., plastic resins, used is quite varied, and the purpose of using this technique can be for aesthetics, for example, providing parts of several colors, technical, for example, providing different technical properties specific for different areas of the part, or economical, for example, manufacturing a substantial area of the part with an economic material and other areas with a quality material, for example, to provide desired finishes or to comply with a predetermined sanitary condition.
Producing hollow bodies of plastic material is known, which bodies are herein generally referred to as “bi-material hollow bodies”, formed by a base layer and a coating layer of different plastic materials. These bi-material hollow bodies include, for example, recipients such as vessels, containers, jars, etc., lids and caps for recipients, and parts for household appliances and vehicles.
Several methods for the production of bi-material plastic parts are known, the main methods being the following.
Overmolding by Means of Insert Transport.
The insert, i.e., the part formed by the first layer of base molding material, is transported from a base molding cavity in which it has been injected to another overmolding cavity in which the coating layer will be injected. This transport can be carried out manually or robotically. The base molding cavities and the overmolding cavities can be in two different molds mounted in two different injector presses or in one and the same double mold.
Overmolding by Means of Half-Mold Rotation.
This system allows overmolding the parts without previously extracting them from their initial mold. The parts are retained in the half-mold (usually on the ejecting side), the half-mold performs a rotation, generally 180°, and the position of the parts that are then overmolded in the overmolding cavities is inverted.
Overmolding by Core Displacement (Also Called Core-Back System).
A core acts as a valve to separate the base molding cavities from the overmolding cavities. First the base layer is molded in a base molding cavity, and then the core opens the passage to the overmolding cavity.
Co-Injection.
The two different materials are molded inside the same molding cavity. The system is based on making the second material pass through the first one to create several layers of different materials.
A particularly important group of bi-material hollow bodies is the group of pre-forms for the production of bottles and other recipients. These pre-forms consist of hollow bodies of plastic material in the form of a tube provided with a mouth and a neck, which optionally includes an external threading and a perimetrical ring-shaped flange. The pre-forms are intended for the later production of plastic bottles by blowing the part in the form of a tube inside a blow shaping mold, in which process the neck and the mouth are unaltered. The production of bi-material pre-forms can be carried out by several of the techniques mentioned above, for example, by overmolding by means of transporting the insert, overmolding by means of half-mold rotation, or co-injection. However, each of these techniques has drawbacks and/or limitations.
In the technique of overmolding by transporting the insert, when the insert is extracted from the base molding cavity, the recently molded base layer forming it is very hot and therefore in a soft state, which involves a risk of undergoing deformation or other damages during transport to the overmolding cavity. Furthermore, the equipment for applying this technique is complex and expensive, and requires a large availability of space given that it generally comprises using two molds, two injector presses, and a robot or other transport means.
In the technique of overmolding by means of half-mold rotation, the base molding and overmolding cavities are located symmetrically in relation to an axis of rotation of the mold. Accordingly, both injection steps must be inscribed in the surface of the injector press limited by the 4 columns of the press. For this reason, the size of the press must be enormous or the number of cavities of the mold very limited. Furthermore, the coolant fluid of the mold, which in the technique of bi-material injection must be abundant, must pass through a rotary joint, which additionally limits the capacity of the system. In addition, the pre-forms must be ejected in a displaced manner, and since the necessary force is very important, the ejecting system tends to be decompensated.
In the technique of co-injection, which is currently widely used, the layer of overmolding material cannot be perfectly delimited, whereby the final geometry of the base layer and the coating layer formed by different materials cannot be controlled exactly. This limitation, even though it allows the technique to be used for applying barrier layers, does not make it suitable for the use of recycled materials in combination with quality materials nor for generating aesthetic bicolor motifs by means of molding and overmolding two materials of different colors.
In another order of things, international patent application PCT 2006/ES 00001, belonging to one of the inventors of this invention, describes an apparatus for injection molding of pre-forms comprising a number n of rows of molding cavities alternately interposed between a number n+1 of rows of cooling cavities, and a number 2n of rows of cores fixed to a core plate adapted and actuated to be moved over the base plate in a transverse direction between two positions, in which the cores are aligned respectively with first and second sets of cavities. Each of said first and second sets of cavities is formed by said number n of rows of molding cavities and a number n of the rows of cooling cavities including one or the other of the end rows of cooling cavities, respectively. The base plate is actuated such that it can move in a transverse direction between a closed position, in which the cores are introduced in said first or second sets of cavities, and an open position, in which the cores are extracted from the first or second sets of cavities. Each core has associated thereto an ejecting element configured to define a part of the mold of the pre-form and actuated to perform a transverse movement along the core and thereby eject the pre-form. The ejecting elements are placed in several rows, each associated to one of the rows of cores. The ejecting elements of each row are connected to an ejecting plate, and the different ejecting plates are actuated independently by means of selecting elements to eject the pre-forms only from those rows of cores which have been extracted from rows of cooling cavities.
In this apparatus, the alternating movement of the core plate in combination with the movements of the base plate allows, in one position of the cores in relation to the molding and cooling cavities, injecting molding material in the molding cavities of pre-forms while at the same time other previously injected pre-forms are cooled in the cooling cavities, and subsequently, inverting the positions of the cores in relation to the molding and cooling cavities, after ejecting the cooled pre-forms, to inject new pre-forms on the recently released cores and simultaneously cooling the pre-forms recently injected in the previous position, and so on cyclically.